Long term evolution (“LTE”) of the Third Generation Partnership Project (“3GPP”), also referred to as 3GPP LTE, refers to research and development involving the 3GPP LTE Release 8 and beyond, which is the name generally used to describe an ongoing effort across the industry aimed at identifying technologies and capabilities that can improve systems such as the universal mobile telecommunication system (“UMTS”). The notation “LTE-A” is generally used in the industry to refer to further advancements in LTE. The goals of this broadly based project include improving communication efficiency, lowering costs, improving services, making use of new spectrum opportunities, and achieving better integration with other open standards.
The evolved universal terrestrial radio access network (“E-UTRAN”) in 3GPP includes base stations providing user plane (including packet data convergence protocol/radio link control/media access control/physical (“PDCP/RLC/MAC/PHY”) sublayers) and control plane (including a radio resource control (“RRC”) sublayer) protocol terminations towards wireless communication devices such as cellular telephones. A wireless communication device or terminal is generally known as user equipment (also referred to as “UE”). A base station is an entity of a communication network often referred to as a Node B or an NB. Particularly in the E-UTRAN, an “evolved” base station is referred to as an eNodeB or an eNB. For details about the overall architecture of the E-UTRAN, see 3GPP Technical Specification (“TS”) 36.300 v8.7.0 (2008-12), which is incorporated herein by reference. For details of the communication or radio resource control management, see 3GPP TS 25.331 v.9.1.0 (2009-12) and 3GPP TS 36.331 v.9.1.0 (2009-12), which are incorporated herein by reference.
As wireless radio communication systems such as cellular telephone, satellite, and microwave communication systems become widely deployed and continue to attract a growing number of users, there is a pressing need to accommodate efficiently a large and variable number of communication devices that transmit an increasing quantity of data within a fixed spectral allocation and limited transmitter power levels. The increased quantity of data is a consequence of wireless communication devices transmitting video information and surfing the Internet, as well as performing ordinary voice communications. Such processes are generally performed while accommodating substantially simultaneous operation of a large number of wireless communication devices.
At present, there are mainly two kinds of wireless communication system or network architectures, centralized and distributed. A centralized communication network can be taken as a conventional infrastructure-based cellular communication network whereas an ad-hoc communication network exemplifies a distributed communication network. In a centralized cellular communication network (also referred to as a primary communication system), a wireless communication device such as user equipment communicates with another wireless communication device such as user equipment through a base station, which is also referred to as primary spectrum usage. However, in an ad-hoc communication network (also referred to as a secondary communication system), the user equipment communicates directly with another user equipment (or through a relay), which is also referred to as secondary spectrum usage. In the primary communication system, traffic goes through a centralized network control element such as a base station even if the source and destination user equipment are close to each other. The main benefit of such operation is easier communication resource and interference control, but the obvious drawback is inefficient communication resource utilization. For example, significantly more communication resources are generally required for cellular communications (or a cellular communication mode) compared to a device-to-device (“D2D”) communications (or D2D communication mode) when the user equipment are relatively close.
Thus, it may be beneficial for spectrum to be opportunistically shared between centralized and distributed communication systems. For example, unlicensed distributed communication devices such as wireless local area network (“WLAN”) communication devices will coexist with overlapping spectrum with licensed users such as cellular communication devices. Interference-aware resource scheduling and dynamic channel allocation may become common practice.
Orthogonal frequency division multiplexing (“OFDM”) is a modulation candidate for sharing a frequency band between centralized and distributed communication devices because the spectrum of the transmitted signal is shaped by assigning or de-assigning subcarriers to a given frequency range. Single-carrier frequency division multiple access (“SC-FDMA”) is another modulation candidate for sharing a frequency band between centralized and distributed communication devices. However, a well-recognized problem in OFDM and SC-FDMA signal generation is the generation of relatively high sidelobes in the transmitted signal.
In conventional radio communication systems, sidelobes are not a major concern. Usually, a limiting factor is linearity of a power amplifier of a transmitter (or transceiver), which results in typical adjacent channel power ratios between −25 dBc (the power ratio in decibels of a signal to a carrier signal) to −45 dBc. A communication system is designed for a worst-case scenario (i.e., the transmitter running at full power), and a sufficient width of guard bands is allocated between adjacent channels, for example, ten percent in a LTE-based communication system or a wireless local area network. Additionally, discontinuities in nearby or adjacent symbols (or symbol waveforms) should be taken into account.
In future communication systems, the rigid channel structure of today's communication systems may not apply. For example, in opportunistic channel reuse, a communication device may detect an opportunity to transmit in a narrow frequency band in the presence of other communication devices operating on nearby frequencies that may not be interfered. Often, such transmit opportunities may be exploited using only a fraction of a maximum possible transmit power of the power amplifier. When the power amplifier is not used at its maximum rated power, distortion products caused by the power amplifier become small and other mechanisms start to dominate unwanted emissions into adjacent channels. One such mechanism is caused by waveform discontinuities at boundaries between symbols, referred to as “sinc leakage” (sinus cardinalis). Filtering as used, for instance, in communication devices for LTE-based communication systems, is inefficient to suppress sinc leakage for discontinuous transmit bands.
Thus, there is need for an improved system and method that can address interference issues for communication devices operable in primary and secondary communication systems that avoid the deficiencies of the present communication systems.